Recent evidence has demonstrated that a variety of human peptide growth factors have associated with their function naturally occurring binding proteins (BP) which modulate the ligand mediated proliferative response. The best example of this interaction is seen with insulin-like growth factor I (IGF-I) and at least ~ve distinct BPs (encoded on different genes) that regulate IGF-I/receptor binding. It has been previously shown that different IGF-I BPs can have dichotomous effects on IGF-I induced growth. Some BPs bind to IGF-I, causing alterations in the ligand's structural conformation that results in enhancing type I receptor af~nity and augmenting IGF-I's proliferative effect; these are superagonists. In contrast, other BPs interact with IGF-I to induce a steric interference of type I receptor binding and thereby block IGF-I effects; these are antagonists. As an extension of our recent report on a newly discovered mitogenic peptide (IBE1 peptide amide) found within the E domain of IGF-IB prohormone, we have investigated whether BPs exist for this ligand. Towards this end, we examined if plasma or serum from normal and diseased individuals (lung cancer patients) contained IBE1 BPs. Initial screening studies utilized 125I-IBE1 as a labeled tracer to determine the presence of BPs. Plasma or serum samples were incubated with the labeled ligand over different time courses (2 hr, 4 hr, 24 hr) and assessed for the presence of BPs by agarose electrophoresis. Alterations in the electrophoretic mobility (EM) of the free ligand over the test samples were interpreted as a positive indicator of BP expression. In normal individuals there were dramatic differences in the electrophoretic profile observed between homologous plasma/serum samples. Plasma universally showed no altered EM shifts over the free-ligand. However, the sera of all 6 normal donors demonstrated the existence of at least three distinct BPs having EMs of 1.4 cm, 2.6 cm and 4.2 cm from the origin. Differences in the binding patterns observed between plasma and serum were not due to the presence of a chelator since NaCitrate at 1x, 2x and 10x concentration used to generate the plasma did not alter serum binding results. These BPs may represent naturally occurring regulators of IBE1 function, a possibility which we are now actively investigating.